Heat transfer experiments were performed for a high-aspect-ratio (˜18) rectangular duct having a sharp-edged inlet, with air being drawn into the inlet from a large upstream space. The experiments encompassed data runs where both of the principal walls of the duct were isothermal (at the same temperature) and other runs where one wall was isothermal while the other was adiabatic. Local heat transfer coefficients were determined for all runs. It was found that flow separation at the duct inlet played a decisive role in shaping the axial distribution of the heat transfer coefficient in the thermal entrance region. Of particular note is a high heat transfer peak at the point of flow reattachment. The peak is situated at an axial station less than one hydraulic diameter from the inlet and moves upstream with increasing Reynolds number. The heat transfer coefficients for symmetric and asymmetric heating are identical in the initial portion of the thermal entrance region. Deviations occur farther downstream but do not exceed more than about 7 percent. The entrance length for asymmetric heating is significantly greater than that for symmetric heating.